An automatic transmission mounted on a vehicle is configured with a transmission mechanism connected to an engine via a torque converter for example and having a plurality of power transmission paths. The automatic transmission is configured to automatically switch the power transmission paths based on the accelerator pedal position and the vehicle speed, for example, namely to automatically shift the gear ratio (gear). Generally, the vehicle having the automatic transmission includes a shift lever operated by a driver and, based on the operation of the shift lever, the shift position is set in any position (for example, rearward drive position, neutral position, forward drive position). In the shift position (usually forward drive position) set in this manner, automatic transmission control is performed.
When the vehicle having such an automatic transmission as described above is set in the forward drive position and the vehicle is stopped, the driving force from the engine which is idling is transmitted via the torque converter to the transmission and then transmitted to wheels, resulting in a so-called creep phenomenon. While the creep phenomenon is very useful under certain conditions, specifically, useful in that the vehicle stopped on a slope can be started smoothly, the creep phenomenon is unnecessary when the vehicle should be kept stopped. In such a case, the brake of the vehicle is activated to suppress the creep force. In other words, the creep force from the engine is suppressed by the brake, resulting in the problem of accordingly deteriorated fuel economy of the engine.
In view of the above, the following proposal has been made. Namely, under the conditions: the vehicle is in the forward drive position; the brake pedal is pressed down to activate the brake; the accelerator pedal is substantially fully released; and the vehicle is stopped, the transmission is set in a neutral state which is close to the neutral while the forward drive position is kept, thereby improving the fuel economy (in the following, this operation is also referred to as “neutral control”). At this time, a certain frictional engagement element of the transmission is disengaged (to be accurate, the engagement pressure of the frictional engagement element is decreased so that the element is in a slip state under control).
Many techniques have been disclosed regarding to make determination about conditions for permission and conditions for inhibition of execution of such neutral control to execute the neutral control, inhibit the neutral control, or make a transition from the state of permission to the state of inhibition (namely return from the neutral control). For example, when the hydraulic fluid of the automatic transmission that operates a frictional engagement element of the automatic transmission has a low temperature, the hydraulic fluid has a high viscosity and thus is highly viscous. Therefore, the response of the frictional engagement element whose engagement and disengagement are controlled by the hydraulic fluid pressure deteriorates. Accordingly, the control when the vehicle is restarted could deteriorate, and therefore, when the vehicle is restarted, the driver could feel that something is wrong. In order to avoid this malfunction, some techniques have also been disclosed.
Japanese Patent Laying-Open No. 11-193866 discloses a neutral control apparatus for an automatic transmission by which the effect of enhancing fuel economy can be further improved and the driver's feeling that something is wrong can be alleviated. The neutral control apparatus for the automatic transmission sets the automatic transmission in the neutral state when predetermined conditions are satisfied, even if the shift position of the automatic transmission is the forward drive position. The apparatus includes means for determining whether or not predetermined conditions are satisfied, specifically (a) means for determining whether or not the value of the accelerator pedal position is at most a predetermined value, (b) means for determining whether or not the brake pedal is pressed down, (c) means for determining whether or not the timing is shift down timing and (d) means for determining whether or not the value of the vehicle speed is at most a predetermined value. The neutral control apparatus for the automatic transmission further includes means for allowing the automatic transmission to make a transition to the neutral state when all of these determination means determine that respective conditions are satisfied. The neutral control apparatus for the automatic transmission still further includes means for inhibiting the transition to the neutral state when the value of the engine coolant temperature is at most a predetermined value or the automatic transmission hydraulic fluid temperature is at most a predetermined value.
Regarding the neutral control apparatus for the automatic transmission, when the brake pedal is pressed down to gradually decelerate and stop the vehicle, the automatic transmission is changed to the neutral state at the downshift timing immediately before the vehicle is actually stopped, namely the timing of downshift to the first gear. Therefore, as compared with the conventional case where the vehicle is driven over a certain distance in the first gear and then the transition is made to the neutral state when the vehicle is stopped, the effect of enhancing fuel economy is further improved since the transition to the neutral state is made earlier. Further, the downshift to the first gear immediately before the vehicle is stopped causes unnecessary shift shock to the driver and further, the transition from the first gear to the neutral state after the vehicle becomes stationary causes the driver to feel that something is wrong. In contrast, the apparatus of the invention does not make a transition to the neutral state while the vehicle is stationary, but makes a transition to the neutral state at the downshift timing before the vehicle becomes stationary. Therefore, even slight shock to the driver can be alleviated. Further, as conditions for making a transition to the neutral state may additionally include the hydraulic fluid temperature and the coolant temperature, and accordingly further stable control can be implemented.
When this neutral control is not performed while the vehicle is temporarily stopped in the forward drive position (D position), the driving force from the idling engine is transmitted via the torque converter to the automatic transmission. The automatic transmission is in the state where the frictional engagement element implements the first gear for example since the shift position is the D position. In this state, the vehicle is stopped by the brake. Therefore, while a pump impeller on the input side (engine side) of the torque converter is rotated, a turbine impeller on the output side (automatic transmission side) of the torque converter is not rotated.
However, when the automatic transmission hydraulic fluid has a low temperature, the oil level in the oil pan is lower and accordingly the hydraulic fluid containing air bubbles is supplied to the inside of the automatic transmission. In this state, if the neutral control is not performed while the vehicle is stooped in the D position, the air bubbles stay or accumulate in the torque converter since the turbine impeller of the torque converter is not rotated. The presence of such air bubbles makes ineffective the power transmission ability of the torque converter or the ability is deteriorated, resulting in decrease in driving force when the vehicle is restarted (lost drive phenomenon).
The neutral control apparatus for the automatic transmission disclosed in Japanese Patent Laying-Open No. 11-193866 as described above inhibits the neutral control when the temperature of the hydraulic fluid of the automatic transmission is low. Therefore, the driver more strongly feels that something is wrong when the vehicle is restarted, due to deterioration of the power transmission ability caused by air bubbles.